Chronic fatigue syndrome (CFS) is a serious health problem in the United States, affecting at least 3 of every 1,000 patients seen in general medicine clinics. It is estimated that the prevalence rate in the United States is 4 - 10/100,000 persons. No effective treatment has been identified. Recent observations suggest a strong association between CFS and a treatable disorder in the regulation of blood pressure known as neurally mediated hypotension. In a small unblinded studies, treatment with fludrocortisone and other medications directed against neurally mediated hypotension has appeared to be beneficial, with 40% of treated patients reporting an almost complete resolution of symptoms and another 30% reporting some improvement. The specific aim of this randomized double blind, placebo-controlled trial is to determine whether fludrocortisone is efficacious for those with CFS. They study will randomize 100 adults with CFS to receive either fludrocortisone or placebo. The patients will be entered either through Johns Hopkins Hospital or through NIH. Before, during, and for 2 weeks after treatment, participants will complete self- assessment forms. The primary indicator of efficacy will be a 15-point improvement (on a scale of 1 to 100) in the general sense of well being score. Eligibility criteria include persons 18-49 years of age who satisfy the 1994 CDC criteria for CFS, and have undergone a medical evaluation to exclude other causes of CFS, and have hypotension provoked during stage 1 or 2 of an upright tilt table test, performed as part of the study. Together with our collaborators at Johns Hopkins, we have screened over 160 subjects by tilt-testing, finding about 53% of them to be abnormal. About 82 of the desired 100 study subjects have been enrolled. The blinded treatment has been well tolerated with no serious adverse reactions, nor concerns of our Data and Safety Monitoring Board.